Method for the production of polyactide from a solution of lactic acid or one of the derivatives thereof

ABSTRACT

A process for the production of polylactide, the stages of which for the production and purification of lactide, starting from an aqueous solution of lactic acid or of its derivatives, includes evaporation of water with formation of oligomers, depolymerization to give lactide, condensation and then crystallization of the crude lactide product to give purified lactide, aqueous treatment of the residual fractions from the crystallization and polymerization of purified and/or prepurified lactide to give polylactide in an extruder and in the presence of catalysts. An alternative process includes carrying out the aqueous treatment before the crystallization.

REFERENCE TO RELATED APPLICATIONS

This application claims priority to PCT Application PCT/EP03/050360filed on Aug. 4, 2003, which claims priority to Belgian PatentApplication BE 2002/0469 filed on Aug. 6, 2002.

INTRODUCTION BACKGROUND OF THE INVENTION

A multitude of processes for the preparation and/or purification oflactide and polylactide (PLA) have been described in the literature todate. However, it has to be observed that, even if their scientificinterest is undeniable, the very great majority of these processesremain laboratory processes which could never be operated industrially.This is because they have recourse either to highly specific (if notunique) equipment having no equivalent on the industrial scale, whichrenders their extrapolation and/or management very hazardous orcomplicated; or to a very low productive output and/or the substantialuse of consumables which prevent any economically profitable operationof the process.

In point of fact, although resulting from a renewable starting material(not dependent on oil) and benefiting from a biodegradability whichmakes it possible to envisage it as one of the solutions to theincreasing problem of waste, PLA will find its welcome only in thecontext of a cost price comparable to those currently available forpolymers of petrochemical origin of the commodity products sector.

Nevertheless, two processes resulting from the state of the art mightmeet these requirements.

The first is disclosed in U.S. Pat. No. 5,274,073 of Gruber et al.

Gruber et al. envisage an integrated process for the synthesis of PLAstarting from a solution (more or less pure) of lactic acid and/or ofone of its esters comprising:

-   1. in one or two stages, evaporation of the free water and of a    portion of the bonded water, so as to produce an oligomer with a    molecular mass of between 100 and 5000 amu;-   2. mixing the depolymerization catalyst with the oligomer, followed    by thermal cracking of the mixture with production of lactide in the    vapor form;-   3. selective condensation of the vapors, followed by fractional    distillation, making it possible to recover a purified lactide; and-   4. polymerization of the purified lactide by ring opening to produce    PLA.

The second is disclosed in U.S. Pat. No. 5,521,278 of O'Brien et al.

O'Brien et al. envisage an integrated process for the synthesis of thepurified lactide for PLA starting from an aqueous lactic acid solutioncomprising at least 50% by weight of lactic acid comprising:

-   1. evaporation of the free water and of a small portion of the    bonded water, so as to produce an oligomer comprising a number of    monomer units (n) of between 2 and 8;-   2. continuing the evaporation characterized by a greater diffusion    surface area for the polymer and making it possible to obtain an    oligomer comprising a number of monomer units (n) of between 8 and    25, stages 1 and 2 being carried out in equipment having a structure    characterized by a low iron content;-   3. mixing the depolymerization catalyst, devoid of alkali metals,    with the oligomer, followed by thermal cracking of the mixture at a    temperature below 240° C. with production (a) of a vapor phase    comprising lactic acid, water, lactide and entrained heavy oligomers    and (b) of a liquid phase comprising the heavy oligomers;-   4. extraction of the fraction in the vapor form (a), so that its    residence time in the cracking region is less than 15 seconds;-   5. selective condensation of the vapors, followed by fractional    distillation, making it possible to recover, by an intermediate    extraction, a prepurified lactide in the liquid form; and-   6. melt crystallization of the prepurified lactide, so as to result    in a purified lactide fraction characterized by a residual acidity    of less than 6 meq/kg.

Although these two processes appear advantageous, they have a number ofshortcomings which might render problematic their chances of being usedin an economic and profitable way for the production of a PLA of thequality for the commodity products sector.

If the teachings of Gruber et al. are considered, it is noticed that, bythis process, the quality of the lactide obtained is not sufficient tomake possible the synthesis of a polymer (PLA) with mechanicalproperties corresponding to those of the various applications selected.This is because it is well known to persons skilled in the art that thelowest possible contents of residual water and residual acidity arerequired in order to obtain polymers of high molecular mass (mechanicalproperties) with a high conversion (mechanical properties, stability,yield) and in a short reaction time (chemical and thermal stability;productive output).

In point of fact, by the purification technology selected, namelydistillation, it is impossible to obtain, first, an optically pureproduct [the vapor pressure curves of the various stereoisomers(L-lactide or L-LD, D-lactide or D-LD, meso-lactide or meso-LD) beingmuch too close, which proves to be essential for applications requiringa degree of crystallinity of the polymer] and, secondly, a chemicallypure product as, by their own admission, they recognise that they cannottotally avoid the opening of the lactide ring in the distillation columnand thus the contamination of the lactide in the system.

If the teachings of O'Brien are considered, it is noticed, following theintroduction of an additional stage, namely the melt crystallization,that the optical and chemical quality of the lactide is achieved.However, the new process recommended consists of an extensive sequenceof different technologies which, first, increases the complexity in themanagement of the process and, secondly, renders problematic itseconomic profitability, both with regard to capital costs and productioncosts. Furthermore, if all the stages of the process (evaporations;thermal cracking and distillation), with the exception of the meltcrystallization, are examined, they are all characterized by highoperating temperatures, which is in contradiction with the rules of theart generally recommended in the context of the synthesis of aheat-sensitive product, such as lactide.

SUMMARY OF THE INVENTION

In the continuation of this text, unless otherwise indicated, thepercentages are expressed by weight and the molecular masses by atomicmass unit (amu). In a first embodiment, the invention consists of alow-temperature integrated process for the production and purificationof lactide starting from an aqueous solution of lactic acid or of lacticacid derivatives, comprising:

-   a) evaporation of the free water and of a portion of the water of    constitution until oligomers having a molecular mass of between 400    and 2000 amu, a total acidity as lactic acid equivalent of between    119 and 124.5% and an optical purity, expressed as L-lactic acid, of    between 90 and 100% are obtained;-   b) feeding the mixture comprising a depolymerization catalyst and    the oligomers obtained in a) to a depolymerization reactor with    production of:    -   b1) a lactide-rich vapor phase, and    -   b2) an oligomer-rich liquid residue;-   c) selective condensation of the lactide-rich vapor (b1) with    recovery, in the liquid form, of a crude lactide product freed from    the volatile compounds;-   d) melt crystallization of the crude lactide product (c), with    production of a purified lactide fraction having a residual acidity    of less than 10 meq/kg, a water content of less than 200 ppm and a    meso-lactide content of less than 1%;-   e) aqueous treatment of the residual fractions from the melt    crystallization, consisting of:    -   e1) extractive and controlled crystallization of these fractions        in an aqueous medium, with control of the geometry of the        crystals formed and with segregation of the lactide suspension        towards the solid phase and of the impurities towards the liquid        phase, so as to carry out aqueous extraction of the impurities;    -   e2) separation of the suspension of crystals (e1) from the        liquid phase and then draining, which separates a wet cake rich        in lactide crystals from a liquid phase depleted in lactide and        laden with impurities;    -   e3) drying the wet cake (e2), which provides the prepurified        lactide.

A second embodiment of the invention consists of a low-temperatureintegrated process for the production and purification of lactidestarting from an aqueous solution of lactic acid or of lactic acidderivatives, comprising:

-   a) evaporation of the free water and of a portion of the water of    constitution until oligomers having a molecular mass of between 400    and 2000 amu, a total acidity as lactic acid equivalent of between    119 and 124.5% and an optical purity, expressed as L-lactic acid, of    between 90 and 100% are obtained;-   b) feeding the mixture comprising a depolymerization catalyst and    the oligomers obtained in a) to a depolymerization reactor with    production of:    -   b1) a lactide-rich vapor phase, and    -   b2) an oligomer-rich liquid residue;-   c) selective condensation of the lactide-rich vapor (b1) with    recovery, in the liquid form, of a crude lactide product freed from    the volatile compounds;-   d) aqueous treatment of the crude lactide product resulting from (c)    consisting of:    -   d1) extractive and controlled crystallization in an aqueous        medium, with control of the geometry of the crystals formed and        with segregation of the lactide suspension towards the solid        phase and of the impurities towards the liquid phase, so as to        carry out aqueous extraction of the impurities;    -   d2) separation of the suspension of crystals (d1) from the        liquid phase and then draining, which separates a wet cake rich        in lactide crystals from a liquid phase depleted in lactide and        laden with impurities;    -   d3) drying the wet cake (d2), which provides a prepurified        lactide;-   e) melt crystallization of the prepurified lactide (d3), with    production of a purified lactide fraction having a residual acidity    of less than 10 meq/kg, a water content of less than 200 ppm and a    meso-lactide content of less than 1%.

However, the invention also proves to be advantageous in the context ofa process for the production of polylactide, the phase of production andof purification of lactide starting from an aqueous solution of lacticacid or of lactic acid derivatives comprising stages a) to e3) of thefirst embodiment above, to which a stage of polymerization of lactide topolylactide is added.

Very clearly, in the context of a process for the production ofpolylactide, a stage of polymerization of lactide to polylactide canalso advantageously be added to the phase of production and ofpurification of lactide starting from an aqueous solution of lactic acidor of lactic acid derivatives comprising stages a) to e) of the secondembodiment above.

In the present invention, the stage of the process which consists of anextractive and controlled crystallization in an aqueous medium oflactide fractions, with control of the geometry of the crystals formedand segregation of the lactide suspension towards the solid phase and ofthe impurities towards the liquid phase, so as to carry out an aqueousextraction of the impurities, exhibits specific characteristics:

-   -   this crystallization is carried out with an amount of water        which is as low as possible (for example, 0 to 25%);    -   during the crystallization phase, the mixture (lactide+water)        will be brought to and maintained at a temperature just below        its crystallization temperature (for example, 5° C. below);    -   the contact time of this mixture will be reduced as much as        possible (for example, 1 to 45 min).

There are many advantages to proceeding in this way:

-   -   large crystals are obtained which exhibit a lamellar structure,        without inclusions or occlusions, and which are pure, stable and        easy to handle;    -   under these conditions, a complex (one molecule of lactide+one        molecule of water) is also formed;    -   no or only a little meso-lactide is removed by hydrolysis;    -   the meso-lactide which is not removed by hydrolysis is thus        recycled; this recycling represents a very worthwhile economic        advantage for the process;    -   the formation of large crystals also promotes the transfer of        the impurities towards the aqueous phase;    -   the formation of large crystals makes it possible to obtain more        efficient subsequent separation and more efficient subsequent        drying.

The formation of these large crystals is an essential indication thatthe process operates under conditions of temperatures, of times and ofamounts of water in accordance with the invention: they are aconfirmation of the fact that the process is indeed managed according tothe invention. These large crystals are formed under conditions opposedto those of bulk crystallizations. The control of the crystallizationaccording to the invention is carried out by control of the profile ofthe temperatures: no sudden fall but the obtaining and then maintenancefor a certain time of a temperature just below the crystallizationtemperature.

By operating in that way, a region of weak supersaturation isencountered which promotes and makes it possible to control the growthof the crystals. In order to further improve this control, the mixtureis seeded with pure lactide crystals in order to reduce as much aspossible the formation of new nuclei.

It is not very easy to quantify the size of these large crystals with alamellar structure. This is because the size can be measured along the 3axes of the crystal but also by the mean value of the 3 measurements.Furthermore, the size of the crystals comes under a statistical andtherefore random phenomenon. The size can also be quantified by passingthrough one or more sieves, by introducing a percentage of crystalswhich pass through the sieve, the remainder being rejected by one sievein particular. Other principles of measurement are also possible. Themeasurement is further complicated when aggregates of crystals resultingfrom an industrial process, rather than isolated and perfectly formedcrystals, are considered. In the case of bulk crystallization, itsometimes appears impossible to define an individual crystal. Finally,it is also necessary to take into account the variations inherent in anyindustrial process, which will cause the sizes to vary although all theparameters of the production process are unchanged.

It appears simpler to quantify the size of these crystals if the matteris examined comparatively. It is clear that the mean size of theindividual crystals obtained by the process according to the invention,whatever the operating conditions but provided that the parametersindicated above (small amount of water added, temperature just below thecrystallization temperature, contact time reduced as much as possible)are observed, is visibly greater than the mean size of individualcrystals obtained by bulk crystallization (following a sudden fall inthe temperature). A few measurements carried out in a rudimentaryfashion during the 2 types of production process make it possible to saythat, if the mean size by bulk crystallization is 0.1 mm, then the meansize by crystallization according to the invention is 0.5 mm or more.This comparison only has meaning, of course, if the chemical compositionof the mixture subjected to the 2 types of crystallization is similar,indeed even identical, at the start. The values given above aretherefore more orders of magnitude than absolute values.

Other advantageous aspects of the process mean that the starting lacticacid derivatives comprise lactic acid esters or a mixture of lactic acidand of one or more lactic acid esters.

It should be noted that, for the implementation of the invention, thecrude lactide product is enriched in prepurified lactide fractionsoriginating from the aqueous treatment of the residual fractions fromthe melt crystallization.

The recycling represents an important aspect of the implementation ofthe invention: the prepurified lactide resulting from the aqueoustreatment can be recycled at any point in the production of purifiedlactide.

Moreover, it should be noted that the content of D-lactide, during theprogression of the process, is controlled by polymerization by ringopening of the prepurified lactide.

Furthermore, it will be noted that, during the progression of theprocess, the prepurified lactide exhibits a residual water content ofbetween 50 and 1000 ppm, a total lactide content of between 70 and 99%,a content of lactic acid and lactic acid oligomers of between 0 and 5%and a meso-lactide content of between 0 and 15%.

Finally, for the advantageous progression of the invention, thepolymerization of the purified and/or prepurified lactide comprises thestages:

-   a) of addition of a catalyst or mixture of catalysts to the lactide;-   b) of addition of optional comonomers, of oligomers, of prepolymers,    of stabilizers, of fillers, of reinforcing agents or polymerization    moderators to the mixture (a) during the initiation of the    prepolymerization and/or during the polymerization in an extruder.

In some embodiments of the invention, the polymerization of the purifiedand/or prepurified lactide does not require prepolymerization.

For the process for the production of lactide according to the inventionor of polylactide according to the invention, it should be noted that,during the production and the purification of lactide, the recycledfractions of lactic acid or of its derivatives are introduced in thepurification stage of the process for the production of lactic acid orof its derivatives.

In contrast to the processes of the state of the art, in this inventionthe entire part intended to extract and to purify the lactide is carriedout at low temperature (less than 105° C.), which is an importantadvantage in the context of an integrated process.

This is because, by operating at low temperature, in addition to anobvious economic advantage, any risk of racemization of the products andthus the formation of D-lactic units is removed. It is clear that, forthe processes of the state of the art, these D-lactic units at a lowconcentration do not constitute a problem for the quality of the finallactide provided that they incorporate in the process a unit forstereospecific purification, such as, for example, melt crystallization.However, in the context of an integrated process, this concentrationwill gradually increase and a dysfunctioning in the various stages ofthe process will be observed. This is because a greater content ofD-lactic units would randomly generate a greater proportion ofmeso-lactide or of D-lactide which, on the one hand would be highlyinjurious to the stability of the streams during the distillation(meso-lactide much less stable) and, on the other hand, would disruptthe satisfactory operation of the melt crystallization due to thepresence of the racemic mixture or D-lactide (the optical quality of thefinal product then no longer being assured). It is therefore essentialfor these high-temperature processes to continuously remove its D-lacticunits, thus reducing the overall yield of the process and, at the sametime, its economic viability.

Another advantage of the low-temperature process is the possibility,because of the stage of extractive recrystallization in aqueous medium,of extracting, from the main stream, the infrequent D-lactic unitsgenerated during the first two stages of the process. This is because,on conclusion of this treatment, it is possible to obtain a lactidecharacterized by a chemical purity sufficient to be able to be used as(co)monomer for the synthesis of PLA but with an optical puritycharacterized by the joint presence of a meso- and L-lactide. This novelapproach makes it possible to envisage a completely integrated and thuseconomically viable process.

Another innovative aspect of the present invention consists in recyclingall or a portion of the hydrolyzed by-products resulting from thevarious stages of the process, such as the evaporation distillates, thedepolymerization residue, the filtrate resulting from the extractionwith water, and the like, not directly at the level of the process ofthe synthesis of the lactide but rather at the level of that of theproduction of lactic acid and more particularly before the stages ofpurification of lactic acid. This is because, by proceeding in that way,a gradual increase in the concentration of impurities, such as aminoacids, proteins, glucides, heavy metals, aldehydes, and the like,present in a small amount in the starting material and which willdisrupt the satisfactory operation (technical and economic) of thevarious stages of the process and the purity of the final product, isavoided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of the low-temperature industrial process for theproduction of polylactic acid starting from an alpha-hydroxylatedcarboxylic acid or one of its esters.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Preferably, the starting mixture will be an aqueous lactic acid solutionobtained by the chemical route (hydrolysis of an ester), by thebiochemical route (fermentation) or by mixing recycled fractions. Itslactic acid concentration can vary from 15 to 100%, it being known thatthe evaporation of this free water will result in additional costs.

The chemical and optical purity of the starting material is essential inorder to provide a high conversion yield in the context of an integratedprocess. This is because an excessively low chemical purity of thelactic acid implies the concentration of impurities in the process,which, on the one hand, disturbs the chemistry related to the synthesisof the lactide (racemization, low purification yields) and, on the otherhand, necessitates the introduction of bleeds, which will affect themass balance of the process. Likewise, an excessively low initialoptical purity will result in the appearance of a relatively largeamount (statistical reality) of the other 2 diastereoisomers of thelactide (meso-lactide; D-lactide), which will only complicate thepurification phase and increase the recycling and bleed streams.Although a food grade currently available commercially might besuitable, the grade corresponding to the “Heat Stable” grade, well knownto a person skilled in the art, with an optical purity ≧95% of L isomerand preferably ≧98%, is preferred.

This aqueous lactic acid solution is concentrated by evaporation so asto extract, in a first step, the free water and, subsequently, a portionof the water of constitution. The removal of this water of constitutionis accompanied by the creation of ester bonds by “polycondensation”reaction, which results in the formation of lactic acid oligomers.

The oligomers synthesized are ideally characterized by a molecular massof between 400 and 2000, a total acidity as lactic acid equivalent ofbetween 119 and 124.5% and a D-lactic acid content of between 0 and 10%.This quality makes it possible to avoid, on the one hand, the problemsrelated to the transfer of highly viscous products and, on the otherhand, an excessively high residual acidity in the product obtained onconclusion of the depolymerization stage (synthesis of the lactide).

The evaporation will be carried out while taking care very particularlyto avoid, on the one hand, excessively high entrainment of lactic unitsin the water vapors extracted and, on the other hand, subjecting thelactic acid and its oligomers to a prolonged thermal stress which wouldpromote racemization reactions.

Several steps can be taken, jointly or otherwise, to avoid a prolongedthermal stress on the product.

The first consists in promoting the rapid extraction of the volatilecompound (water) from the reaction medium, so as to shift the reactionbalance towards the formation of oligomers and thus to reduce thereaction time. Entrainment of the volatile compound by vacuum and/or bygas stream constitute advantageous options for carrying out this stage.

A second consists in increasing the reaction kinetics and thus inreducing the reaction time by the addition of an esterificationcatalyst. As the catalyst is of acid type, various acids can beenvisaged. However, care will probably be taken not to use acids ofLewis type (PTSA, ZnCl₂, Ti isopropyl, and the like). This is becausethey act at a level of the hydroxyl group carried by the chiral carbonof the lactic acid and thus can promote racemization reactions byactivating a nucleophilic substitution with inversion of configurationon the methine group. In contrast, protonic acids of H₂SO₄, H₃PO₄, andthe like, type can be used as they act on the oxygen of the carbonylgroup, which ought under no circumstances to promote racemizationreactions. Given the acidic nature of the starting material, thecatalyst can be added during the process, that is to say when theresidual free acidity of the oligomer will no longer be sufficient toeffectively activate the reaction. Depending on the type of acidselected as catalyst, neutralization can be envisaged so as to avoiddecomposition of lactide during the depolymerization stage.

The reaction kinetics are strongly influenced by the temperature.However, the latter also promotes racemization reactions, which have tobe avoided at all costs. In this context, the use of temperatures ofless than 190° C. and of a reactor which can operate under vacuum orunder gas streams and which offers a large exchange surface area and alarge extraction volume will make it possible to solve the problem.

This is because the large exchange surface area will make it possible toprovide, in the minimum of time, the energy necessary for the reactionwhile avoiding overheating, while the large extraction volume willpromote the removal of the volatile compound (water) and thus thereaction kinetics. In this context, different reactors can beadvantageous alternatives, such as, for example, falling filmevaporators, forced circulation evaporators, agitated film evaporators,with or without an internal condenser, and the like.

This phase of the process can be envisaged in one or more stages inorder to optimize the technology with regard, on the one hand, to theviscosity of the streams present, on the other hand to the lactic acidcontent present in the distillates and, finally, to the possible need toadd an esterification catalyst in order to redynamize the synthesis.

The second stage consists of a catalytic and thermal depolymerization ofthe oligomers obtained above, so as to produce a vapor phase rich inlactide. The use of a catalyst proves to be essential in order to reducethe thermal cracking temperature and to avoid the chemical and opticaldeterioration in the lactide synthesized. The catalyst will be solid orliquid and of Lewis acid type, such as, for example, tin octoate, tinlactate, antimony octoate, zinc octoate, and the like. Its content isbetween 0.1 and 5 g %. Catalysts of Lewis acid type are characterized byrelatively high charge density. In point of fact, it has beendemonstrated that these densities promote racemization reactions. Inthis context, it is preferable to reduce as much as possible the contacttime between the catalyst and the oligomers; care will thus be taken tomix the catalyst immediately before it is introduced into the reactor.

For the same reasons, the reactor will be selected so as to maintain the(oligomer/catalyst) mixture for the least time possible (0 to 30 min andpreferably 0 to 15 min) at the reaction temperature while offering alarge exchange surface area and a large extraction volume. The operatingtemperature will be sufficient to initiate the reaction but notexcessively high, in order to avoid decomposition or racemization of thelactide: the temperature will be between 180 and 250° C. The temperatureoptimum will depend on the nature of the starting oligomer (120 to125%), the nature of the catalyst and the pressure in the system.

Given the chemical instability of the lactide at the operatingtemperatures and in order to shift the equilibrium of the reactiontowards the formation of the lactide, it is important to extract it asrapidly as possible from the reaction medium. In this context, it ispreferable to maintain the reaction medium under a gas stream and/orunder vacuum. The second option will be preferred as it also makes itpossible to reduce the reaction temperature.

As a result of the various constraints mentioned above, the use of anevaporator of thin layer type, such as, for example, a thin filmevaporator, seems particularly relevant. This is because a liquidresidue composed of oligomers with high molecular masses is extracted atthe bottom from this type of device. This residue will be recycled afterhydrolysis, care being taken to carry out a pretreatment or a bleedwhich makes it possible to remove the deactivated catalyst.

At the top, the vapor phase rich in lactide is directly extracted andselectively condensed in a condenser maintained at a very specifictemperature. This is because the condenser is maintained at atemperature such that, on the one hand, the volatile compounds, such aswater, most of the lactic acid and decomposition products resulting fromthe synthesis (acetaldehyde, and the like), remain in the vapor phase(while the lactide and the heavy compounds are condensed) and not toolow, on the other hand, to avoid crystallization of the lactide.Depending on the nature and on the purity of the product collected(crude lactide product), this temperature will be between 70 and 125° C.

On conclusion of this selective condensation, there is present a crudeproduct characterized by an L-LD content of greater than 85%, indeedeven 90%, a meso-LD content of less than 7%, indeed even of less than5%, or even of less than 3%, and a residual water content of less than1000 ppm, indeed even of less than 500 ppm.

The subsequent stage of the process consists of a purification of thecrude product in order to obtain a lactide with a chemical andstereospecific purity sufficient for the synthesis of PLA by ringopening. A sufficient purity implies a content of a lactide of between99.0 and 99.9% and more preferably between 99.5 and 99.9%, a meso-LDcontent of between 0 and 1% and preferably between 0 and 0.5%, a watercontent of between 0 and 200 ppm and preferably between 0 and 50 ppm,and an acidity of between 0 and 10 meq/kg and preferably between 0 and 1meq/kg.

The technology of the melt recrystallization (one or more stages) makesit possible to achieve this quality while operating at low temperature.In the context of this technology, the impure lactide obtained above ismelted and subjected to controlled cooling in order to initiate thecrystallization. The impurities will be concentrated in the liquidphase. After the crystallization, the liquid phase is removed bygravity, leaving crystals coated with a film of impurities. In order toremove it, a partial remelting is carried out. The liquid thus obtainedentrains the film and is removed by gravity. The operation is repeateduntil the required purity is achieved. This sequence of stages can be ofthe static and/or dynamic type. The desired purity is achieved, thecontents of the crystallizer are melted and recovered.

However, the profitability of this purification stage is related to theconcentration of L-LD, to the nature of the chemical impurities of thefeed and to the concentration of L-LD present in the residues from thestage.

This is because the nature of the impurities present in the startingfeed directly influences the effectiveness of the purification. Thus, amore viscous impurity will be extracted with greater difficulty and willrequire several purification stages. Likewise, the presence of acidicand aqueous impurities will promote the opening of the lactide ring,which will have a direct consequence on the yield of the stage.

Moreover, the concentration of L-LD in the starting solution makes itpossible to significantly improve the yield by weight (fewer impuritiesto be extracted, less decomposition) but also the profitability (fewerpurification stages). Thus, according to a person skilled in the art,while the theoretical yield for purification by melt crystallizationwith a feed comprising 85% of L-LD is 78.5%, it changes to 86.4% for afeed comprising 90% of L-LD.

However, when the theoretical purification yield is taken into account,it is also necessary to include therein the concentration factor of L-LDin the residue. This is because, in the context of an integratedprocess, it is preferable to be able to recycle the residue as lactideupstream in the process (for example, to enrich a fraction) in order toavoid having to recycle it as lactic units (resulting from thehydrolysis of the lactide), which increases the probability of thermaldecomposition (increased residence time in the process) and results in asignificant energy cost. In point of fact, in order to be able torecycle it in the melt crystallization as lactide, it is necessary forits concentration to be sufficiently high (that is to say, of the orderof 70%) so as to be able to mix it with a richer intermediate fractionand to reintroduce it into the main process stream and if possible theclosest to the final purification stage. Under these conditions,starting from a feed comprising 85% of L-LD, the theoretical yield forpurification by melt crystallization is 78.5%, if the L-LD content ofthe residue is 55%, but it falls to 58.9% if the content in the residueis 70%. Furthermore, with the residue comprising an L-LD charge of 70%,it will be necessary to feed the unit with 200 kg of 85% product inorder for the 100 kg of finished product to emerge, whereas only 150 kgof 85% product is necessary with the 55% residue. On taking into accountthese considerations, it is obvious that this type of technology cannotbe used economically and on an industrial scale without aprepurification stage (such as, for example, distillation) which willmake it possible to substantially increase the purity (the L-LDconcentration) of the starting feed.

For this invention, an important aspect is the incorporation in theprocess of a technology which makes it possible to recover the lactide,in the lactide form and not in the form of a lactic unit, from a residuewith a low charge (for example of the order of 40%), the minimum beingrelated to the presence of a eutectic. In this context, the meltrecrystallization can be run in a different way and makes it possible toobtain, for a feed comprising 88% of L-LD, a theoretical purificationyield of 87% while requiring a feed charge of only 132 kg per 100 kg offinished product. On taking into account these new considerations, it isobvious that this type of technology can, at this time, be usedeconomically and on an industrial scale without a high-temperatureprepurification stage.

The technology which makes possible the recovery of the lactide from theresidue from the melt recrystallization stage will preferably ensurethat:

-   -   either a lactide with a purity sufficient to be able to be        treated by melt recrystallization is produced, that is to say        with an L-LD content equal to or greater than 60% and with a        residual water content which is sufficiently low (<1000 ppm and        preferably less than 400 ppm) to prevent rapid deterioration of        the lactide;    -   or a lactide with a sufficient chemical purity to be able to be        used directly as monomer for the synthesis of PLA by ring        opening is produced.

The technology preferably considered for the extraction of lactide fromthe residue in the context of this invention comprises the followingstages:

-   1. extractive and controlled crystallization of the residue from the    melt crystallization (residues) in an aqueous medium while    controlling the geometry of the crystals formed and while bringing    about phase segregation between the lactide (solid phase) and the    impurities (liquid phase), promoting aqueous extraction of the    impurities;-   2. separation of the suspension of crystals which is obtained in 1    into a liquid phase depleted in lactide and laden with impurities    and into a wet cake rich in lactide crystals;-   3. drying the wet cake obtained in 2.

As this technology is not stereospecific, the product resulting fromthis stage can attain a very high chemical purity and can also comprisea certain content of meso-lactide, which constitutes a very advantageousmethod for the extraction of the D-lactic units from the process. Theproduct thus obtained could be used as additive and mixed with thepurified lactide in order to control the content of D-lactic unitspresent and thus to vary the properties of the polymer synthesized.

The lactide, purified and prepurified, synthesized by the processdescribed in the context of this invention can subsequently either beused as additive for food applications (for example: agent whichcoagulates animal or plant proteins, preservative or pH regulator,raising agent for dough in breadmaking) or can be polymerized by ringopening by a large range of catalysts, including organometallicderivatives of transition metals (Groups 3 to 12) or metals from Groups13 to 15.

A favorite approach of the present invention is the continuouspolymerization of the purified lactide by virtue of the addition of thetin octoate/triphenylphosphine pair to a twin-screw extruder (reactiveextrusion).

Although a single reactive extrusion stage is sufficient to succeed insynthesizing, starting from the lactide, a PLA having mechanicalproperties sufficient to be able to be used in the field of packagingand commodity products, this prospect can result in the followingdisadvantages:

-   -   a low throughput/productive output (prohibitive depreciation        costs);    -   a low stability range for the management of the production units        (situation of precarious balance).

This productive output is related to the nature of the starting materialfed to the twin-screw extruder. This is because the lactide is fed to anextruder which is maintained at a temperature far above its meltingpoint. In point of fact, beyond this melting point, its viscosity isvirtually zero. Consequently, a large part of the reactor is usedoutside its optimum operating conditions for:

-   -   degassing and bringing the starting material to reaction        temperature (heating by calender) during the feeding of a        lactide in the solid form;    -   promoting the propagation of the synthesis (homogenization of        the mixture).

It is only when the viscosity is sufficient that the extruder caneffectively accelerate the reaction by virtue of optimum mixing at highviscosity and by an additional energy contribution related to frictionalphenomena.

Moreover, the absence of viscosity in the first part of the machinerenders the system much more sensitive to possible fluctuations in oneof the production parameters (feed flow rate, catalyst concentration,viscosity (pressure) at the die head, and the like).

In this context, it would be judicious to include therein a first stageof continuous prepolymerization which would be carried out in anyreactor capable of:

-   -   melting (and degassing) the lactide;    -   adding the active principle (catalyst and optionally cocatalyst)        and the optional additives (comonomers, oligomers, prepolymers,        stabilizers, fillers, reinforcing agents, polymerization        moderators);    -   homogenizing the active principle and the additives in the        molten lactide and maintaining the mixture at the polymerization        temperature;    -   initiating the polymerization so as to obtain a product with a        viscosity sufficient to be able to be effectively treated during        the second stage (molecular mass between 5000 and 50 000);    -   feeding the product continuously to an extruder with the        possibility of optionally adding other additives (comonomers,        oligomers, prepolymers, stabilizers, fillers, reinforcing        agents, polymerization moderators) and of homogenizing them.

The second stage will be carried out in a twin-screw extruder.

Moreover, as the synthesis in an extruder is continuous, it would bepreferable, in order to ensure that the PLA produced is homogeneous,also to carry out the prepolymerization stage continuously. In thiscontext, once the lactide is molten (and degassed), technologies ofstatic mixer-heat exchangers (SMXL type from Sulzer or equivalent),static mixer-reactors (SMR type from Sulzer or equivalent) or twin-screwreactors of List ORP® or List CRP® type might be perfectly suitable. Theadvantage of this type of technology is, inter alia:

-   -   the narrow distribution in the residence times (homogeneity of        the product fed to the twin-screw extruder, narrow        polydispersity);    -   the high mixing and dispersing effectiveness for fluids of high        viscosity or having a large viscosity difference (homogenization        of the catalyst or additives in the monomer);    -   the high heat exchange capacity (to accelerate or control the        reaction).

The PLA produced in the context of this invention will either be ahomopolymer (for example, synthesis starting from pure L-lactide) orwill be a copolymer (for example, synthesized starting from lactidecomprising a proportion of meso-lactide or of additives).

Notes:

-   1. The process described regards lactic acid as starting material.    However, this sequence of stages can certainly be applied to lactic    acid esters, such as methyl lactate, ethyl lactate, isopropyl    lactate, butyl lactate, and the like.-   2. In the context of the use of a lactic acid ester as starting    material, the oligomerization stage will necessarily require the use    of an acid transesterification catalyst of para-toluenesulphonic    acid (PTSA), tin octoate, sulphuric acid, and the like, type.-   3. The process described considers solely the L isomer of lactic    acid but it is obvious that it can also be taken into account for    the other isomer, namely D-lactic acid.

A preferred description of the process forming the subject-matter of thepresent invention is described below with reference to FIG. 1.

The aqueous lactic acid solution is fed via the line 1 and can be mixedcontinuously with the hydrolysed liquor fed via the line 2001 andoriginating from the hydrolysis vessel 2000. However, a preferred optionconsists in recycling the hydrolysed liquor directly in the purificationstages of the process for the production of lactic acid via the line2002, so as to be able to remove the impurities, such as amino acids,proteins, metal ions, and the like, therefrom. A hydrolysis residue,preferably in solid form, can be removed via the line 2003, which makesit possible to bleed the system of insoluble product. The hydrolysisvessel is represented only diagrammatically by a vessel but severalvessels can be envisaged, depending on the concentration and on thedestination of the recycled fractions.

The mixture is fed continuously via the line 2 to a preheater 100 whichbrings the mixture to the temperature required for the evaporation ofthe water, that is to say between 50 and 150° C. It is possible tocontinuously add to the mixture, via the line 121, an esterificationcatalyst stored in the vessel 120. During the addition of a catalyst,the preheater 100 will preferably be designed so as to be able to heatand homogenize the mixture. In the evaporator 200, which can operateunder vacuum, at atmospheric pressure or under slight pressure, themajority of the free water and a portion of the water of constitutionare continuously removed in the vapor form via the line 202 andcondensed 210. Depending on the content of lactic acid in thecondensates, the latter are conveyed either to the hydrolysis vessel2000 via the line 211 or as back-up water to the extractivecrystallization vessel 700 via the line 212 or very simply dischargedvia the line 213.

The concentrated lactic acid continuously removed via the line 201 andcharacterized by an average molecular mass of between 100 and 600 is fedcontinuously to a preheater 250 which brings the concentrated lacticacid to the oligomerization temperature, that is to say between 80 and180° C. It is possible to add to the mixture, via the line 261, anesterification catalyst stored in the vessel 260. During the addition ofa catalyst, the preheater 250 will preferably be designed so as to beable to heat and homogenize the mixture. In the oligomerization reactor300, which can operate under vacuum, at atmospheric pressure or underslight pressure, a small amount of free water and a predominance ofwater of constitution are removed in the vapor form via the line 302 andcondensed 310. The condensates are conveyed to the hydrolysis vessel2000 via the line 311. This stage will preferably be carried out undervacuum, without, however, reaching a pressure of less than 40 mbarabsolute, so as to accelerate the reaction kinetics and to reduce theoperating temperature while avoiding the production of an excessivelylarge amount of the cyclic dimer.

The oligomers removed via the line 301 and characterized by a molecularmass of between 600 and 2000 are fed continuously to a preheater/mixer400. This preheater/mixer makes possible the homogenization of thedepolymerization catalyst fed continuously, at a concentration rangingfrom 0.2 to 5%, via the line 521 and stored in the vessel 520 and makesit possible to bring the oligomers/catalyst mixture to a temperature ofbetween 150 and 250° C. (the exact temperature depending on themolecular mass of the oligomers). A neutralizing agent may have to beadded to the oligomers in order to interrupt the activity of theesterification catalyst before incorporation of the depolymerizationcatalyst but this stage has not been represented in FIG. 1. It is alsopossible for the catalyst added in the oligomerization stage to besuitable for the backbiting reaction and, in this context, any additionof catalyst is reduced, indeed even superfluous.

The catalytic depolymerization reactor 500, which is fed via the line401 with the oligomer/catalyst mixture, is managed so as to promote thebackbiting reaction which generates the lactide. In this context, thetemperature will be between 180 and 250° C., the pressure between 0.1and 40 mbar absolute and the residence time of the mixture under thereaction conditions between 0 and 30 min, preferably between 0 and 15min. The following are removed from the depolymerization reactor 500: onthe one hand, a liquid residue (liquid at the operating temperature)rich in oligomers, which is conveyed via the line 502 to the hydrolysisvessel 2000, and, on the other hand, a vapor phase rich in lactide viathe line 501.

The liquid residue collected as bottoms from the reactor ischaracterized by an average molecular mass equal to or greater than thatof the starting mixture 401 and by a concentration of catalyst greaterthan that of the starting mixture 401.

The vapor phase removed at the top of the reactor 500 and rich inlactide 501 is selectively condensed in a condenser 510, so as tomaintain the volatile compounds, such as water, lactic acid anddecomposition products resulting from the synthesis, and the like, inthe vapor form 513 and to recover the lactide and the heavier compoundsin the liquid form (crude lactide product) 511. On conclusion of thisselective condensation, the crude lactide product is characterized by anL-LD content of greater than 85%, indeed even of greater than 90%, a lowmeso-LD content of less than 7%, indeed of less than 5% and indeed evenof less than 3%, and a residual water content of less than 1000 ppm,indeed even of less than 500 ppm. The condensation temperature iscarefully adjusted according to the pressure prevailing in the systemand so as to avoid solidification of the lactide. It will be between 70and 125° C. The volatile compounds removed via the line 513 arecondensed in their turn 550 and are transferred, via the line 551, tothe hydrolysis vessel 2000.

The liquid crude lactide product is fed via the line 511 to a meltrecrystallization unit 600 where the purification is carried out in oneor more steps according to a static and/or dynamic process at lowtemperature, less than 105° C., so as to recover, via the line 601, apure lactide in liquid form. The latter is characterized by a lactidecontent of between 99.0 and 99.9% and more preferably between 99.5 and99.9%, a meso-LD content of between 0 and 1% and preferably between 0and 0.5%, a water content of between 0 and 200 ppm and preferablybetween 0 and 50 ppm, and an acidity of between 0 and 10 meq/kg andpreferably between 0 and 1 meq/kg. During this purification stage, twotypes of residues are generated. The first, removed via the line 603,comprises a sufficient residual L-LD content for it to be able to bemixed with the crude lactide product resulting from the selectivecondensation stage via 511. A sufficient residual L-LD content isregarded as being between 60 and 99%. The second residue (drain) removedvia the line 602 comprises a residual L-LD content of between 80% and35% and is conveyed in the liquid form to the extractive crystallizationunit 700.

In this unit, the drain is mixed with an aqueous phase fed via the line702 with a water content which can range from 0 to 40%. As alreadystated, the aqueous phase fed can originate from the condensates fromthe evaporation stage via the line 212 or at least in part from thesubsequent stage of drying the prepurified lactide via the line 904. Thetemperature of the mixture is subsequently reduced so as to avoidexcessively great supersaturation, so as to control the geometry of thecrystals formed and to promote phase segregation between the lactide(solid phase) and the impurities (liquid phase).

The suspension of crystals thus obtained is subsequently transferred,via the line 701, to a solid/liquid separation unit 800 in order toobtain, on the one hand, a liquid phase depleted in lactide and ladenwith impurities, which will be conveyed via the line 802 to thehydrolysis vessel 2000. On the other hand, a wet cake rich in lactidecrystals is recovered, which cake is characterized by a free watercontent of between 0 and 10%, a total lactide content of between 60 and99%, a content of lactic acid and lactic acid oligomers of between 0 and5%, and a meso-lactide content of between 0 and 15%.

The wet cake is subsequently fed, via the line 801, to a low-temperaturedryer 900 (temperature of the product less than 45°), in order toprevent the meso-lactide from melting, which will make it possible toreduce the residual water content and to bring it to a value of between1000 and 50 ppm. Depending on the purity of the prepurified lactideremoved from the dryer via the line 901 and liquefied in a reheater 910,from where it will be removed via the line 911, it will either be mixed,via the line 913, with the product fed to step 1 of the meltrecrystallization stage or fed directly to one of the intermediate stepsof the melt recrystallization stage (not represented) or, finally,mixed, via the line 912, with the purified L-LD resulting from the meltrecrystallization stage 601 in order subsequently to be polymerized. Themixture between the prepurified lactide 912 and the purified L-LD 601will be adjusted so as to control the content of D-lactic unit(originating from the meso-lactide) present in the final polymer.

The purified L-LD 601 or the mixture of purified L-LD 601 and ofprepurified lactide 912 is mixed with an active principle and brought tothe polymerization temperature, which can be between 120 and 220° C., ina prepolymerization reactor 1000. The active principle or catalyst isstored in the vessel 1020 and is fed via the line 1021. Itsconcentration will be managed so as to maintain the monomer/catalystratio between 500 and 10 000, the exact content depending on the type ofpolymer desired. The catalyst mentioned above can also correspond to themixture of a catalyst with a cocatalyst, such as, for example, the tinoctoate/triphenylphosphine pair. The product resulting from theprepolymerization reactor may already consist of a prepolymercharacterized by a molecular mass of between 10 000 and 50 000. Thelatter is fed via the line 1001 to a polymerization reactor 1100, whichwill preferably be of twin-screw extruder type, in order to continue andbring to completion the polymerization. The polymer resulting from thisstage 1101 is characterized by a molecular mass which can be between 40000 and 350 000 and a conversion of greater than 95%, indeed even ofgreater than 98%. Whether this is before the mixer/exchanger or at thepolymerization reactor, comonomers, copolymers or additives (heatstabilizer, catalytic deactivators, filling or reinforcing components)can be mixed with the lactide stream but this approach is notrepresented in FIG. 1.

Another favored approach of the present invention consists in conveyingthe crude lactide product 511 resulting from the selective condensationvia the line 512 to the extractive crystallization unit 700. In thisunit, the crude lactide product is mixed with an aqueous phase fed viathe line 702 with a water content which can range from 0 to 40%. Thetemperature of the mixture is subsequently reduced so as to avoidexcessively great supersaturation, so as to control the geometry of thecrystals formed and to promote phase segregation between the lactide(solid phase) and the impurities (liquid phase).

The suspension of crystals thus obtained is subsequently transferred,via the line 701, to a solid/liquid separation unit 800 in order toobtain, on the one hand, a liquid phase depleted in lactide and ladenwith impurities, which will be conveyed via the line 802 to thehydrolysis vessel 2000. On the other hand, a wet cake rich in lactidecrystals is recovered, which cake is characterized by a free watercontent of between 0 and 10%, a total lactide content of between 60 and99%, a content of lactic acid and lactic acid oligomers of between 0 and5%, and a meso-lactide content of between 0 and 15%.

The wet cake is subsequently fed, via the line 801, to a low-temperaturedryer (900) which will make it possible to reduce the residual watercontent and to bring it to a value of between 1000 and 50 ppm.

Depending on the purity of the prepurified lactide removed from thedryer via the line 901 and liquefied in a reheater at 910, from where itwill be removed via the line 911, it will either be fed to the meltrecrystallization stage via the line 911 or mixed, via the line 912,with the purified L-LD resulting from the melt recrystallization stage601 in order subsequently to be polymerized.

The liquid prepurified lactide is fed via the line 911 to a meltrecrystallization unit 600 where the purification is carried out in oneor more steps according to a static and/or dynamic process at lowtemperature, less than 105° C., so as to recover, via the line 601, apure lactide in liquid form which is characterized by a lactide contentof between 99.0 and 99.9% and more preferably between 99.5 and 99.9%, ameso-LD content of between 0 and 1% and preferably between 0 and 0.5%, awater content of between 0 and 200 ppm and preferably between 0 and 50ppm, and an acidity of between 0 and 10 meq/kg and preferably between 0and 1 meq/kg. During this purification stage, two types of residues aregenerated. The first, removed via the line 603, comprises a sufficientresidual L-LD content for it to be able to be mixed with the crudeproduct resulting from the selective condensation stage via 511. Asufficient residual L-LD content is regarded as being between 60 and99%. The second residue (drain) removed via the line 602 comprises aresidual L-LD content of between 80% and 35% and is conveyed in theliquid form to the extractive crystallization unit 700.

EXAMPLES a) This Example Illustrates the Importance of the Recycling ofthe By-Products from the Synthesis of the Lactide in the Plant for thePurification of the Lactic Acid and not in the Oligomerization Stage

A stock of lactic acid oligomers was fed to a depolymerization unit atan interval of one month in order to confirm or invalidate the constancyof the results and thus the possibility of recycling the D-lactic unitsdirectly in the process of the synthesis of the lactide.

During the storage period, the oligomer was kept fluid in a closedchamber with stirring and at a temperature of 140° C.

For the depolymerization, the oligomer is mixed with 2% of its weight oftin octoate and fed (25-30 kg/h) to a thin film evaporator maintained at235° C. and with a surface area of 2 m². The vapor generated (impurelactide, crude product) is condensed and the product obtained weighed inorder to determine the productive output of the system but also analysedin order to determine the selectivity thereof.

TABLE I Characterization of the oligomer and of the effectiveness of thedepolymerization Freshly Ageing for prepared 1 month Characterization ofthe oligomer Free acidity (g %) 9.8 10.5 Total acidity (g %) 122.4 122Content of L isomer (%) 97.6 90 Appearance Amber Very dark brown (black)Characterization of the effectiveness of the depolymerization Productiveoutput (kg/h) 24 10 Conversion (%) 80 40 Content of L-lactide (g %) 87.877.9 Content of meso-lactide (g %) 4.8 14.8

On the basis of the results shown in Table I, it is very clearly noticedthat the optical quality and the productive output markedly decline.Prolonged maintenance of the lactic units at a relatively hightemperature results in a gradual decomposition of the latter. Thedecomposition products generated have a strong disrupting influence onthe reaction for the synthesis of the lactide. In this context, therecycling, in the oligomerization, of the lactate units present in theby-products which have been subjected to thermal stress withoutpreliminary purification risks having a strong disrupting influence onthe productive output of the process.

b) Example Demonstrating the Importance of the Incorporation of theExtraction with Water on the Yield of the Process

A stirred reactor heated using 2 electrical resistors (1.2 kW and 2.3kW) was fed with 20 liters of lactic acid sold by Galactic under the“heat-stable” label and characterized by a concentration of 90% and acontent of L isomer of 97.6%. The temperature of the heating resistorsand in the liquid is regulated so as to avoid any difference of greaterthan 20° C. and to prevent the maximum temperature from exceeding 160°C. In order to facilitate the rapid extraction of the volatile compound,the unit is placed gradually under vacuum, the pressure varying betweenatmospheric pressure and 150 mbar. In order to avoid excessively greatentrainment of lactic acid in the distillates, the reactor is surmountedby a column with a height of 0.90 m and a cross section of 0.09 m filledwith Raschig rings (10×10 mm). A temperature probe placed at the columntop makes it possible to monitor the temperature of the vapors and, ifnecessary, to reduce the heating power in order to prevent excessivelygreat entrainment.

After reacting for 7 h, 6.3 kg of distillates characterized by a totalacidity of 3.3% were collected, along with 17.4 kg of an oligomercharacterized by a total acidity of 122.2%, a molecular mass of 1345 anda content of L isomer of 97.3%.

3% by weight of tin octoate are added to the oligomer obtained above andkept stirred at a temperature of 120° C. The mixture is fed at a flowrate of 3 kg/h to a thin layer evaporator of thin film type made ofstainless steel 316 with a surface area of 0.2 m², the walls of whichare heated by circulation of oil, the temperature of which is maintainedat 220-230° C. The vapors generated are condensed in a condenser with asurface area of 1 m² made of stainless steel 316, the temperature of theliquid “coolant” of which is maintained between 80 and 90° C. The entireunit is run under a pressure of between 5 and 10 mbar absolute. Thecrude lactide product is collected at the outlet of the condenser at aflow rate of 2.45 kg/h, has a content of L-lactide varying between 85and 92% and has a content of meso-lactide varying between 3 and 7%.

A sample of the crude lactide product obtained above (800 g), comprising86.4% of L-LD, 4.8% of meso-LD and a residual acidity of 310 meq/kg, isintroduced into a crystallizer composed of a vertical tube made ofstainless steel with a length of 1 m and a diameter of 30 mm. The jacketof the tube is fed with heat-exchange fluid via athermostatically-controlled heating unit for the control of thecrystallization, sweating or remelting phases. This crude product ismelted at 105° C.

Subsequently, the crystallization is initiated on the wall by a gradualreduction in the temperature of the heat-exchange fluid present in thejacket. To prevent occlusions and inclusions in the pure crystals, thisfall in temperature will be from 2 to 8° C./h. A portion of the crudeproduct is crystallized on the wall, whereas the central part includesthe liquid phase (drain) comprising the majority of the impurities.

Once the heat-exchange fluid has been brought to 60° C., the liquidphase is removed by gravity.

The crystals are still covered with a film of impurities which has to beremoved by the sweating stage: the surface of the tube will be heatedvery gradually (from 60 to 98° C.) so as to cause the surfaces of theless pure crystals to melt as their melting point is lower than that ofthe pure product.

Finally, the crystallizer is brought (at 10° C./min) to the meltingpoint of the product (97-102° C.) in order to liquefy all the product,collected by gravity (melt).

A final product, which has to meet the specifications of a lactide forthe synthesis of the PLA, will be subjected to several successive stepsof purification by the same procedure.

The enriching of the intermediate fractions and the overall yield (Yd)by weight of L-LD of the operation are shown in Table II.

TABLE II Step 1 Step 2 Step 3 Step 4 Feed Melt Drain Melt Melt Melt L-LD(%) 86.4 97 55.9 99 99.8 99.9 meso-LD (%) 4.8 2 10.3 0.7 0.2 0.1 Acidity(meq/kg) 520 120 — 56 18 7 Yd of L-LD (%) 100 — 19 — — 81

The L-LD and meso-LD contents are determined by GC after silylation ofthe carboxyl compounds. The acidities are assayed by potentiometry withtetrabutylammonium hydroxide (TBAH) in an anhydrous solvent. The watercontents were determined by Karl Fisher.

The drains resulting from the first steps were mixed so as to obtain amixture comprising 55.9% of L-LD and 9.8% of meso-LD. This product willbe subjected to a prepurification, before being mixed with the productresulting from the depolymerization for purification by meltrecrystallization in accordance with the procedure described above.

25% by weight of cold water are added to 750 g of crude product at 90°C. The mixture is brought rapidly to its crystallization temperature andwill remain there for 30 min in order to promote the nucleation and thenthe growth of the crystals (by seeding with pure lactide crystals).Subsequently, the temperature is gradually reduced to 25° C.

The mixture is subsequently centrifuged at 1500 revolutions/min and 367g of large white crystals (of approximately 0.4 mm) are collected anddried under vacuum at 45° C. The analysis of this product after dryingappears in Table III.

TABLE III After drying L-LD (%) 93.8 meso-LD (%) 6.1 Water (ppm) 440

The dried product resulting from this treatment, after being mixed withthe product resulting from the depolymerization, will be subjected toseveral steps of purification by melt recrystallization in accordancewith the procedure described above.

Table IV shows an increase in the yield and in the effectiveness of themelt purification.

TABLE IV Step 1 Step 2 Step 3 Step 4 Feed Melt Drain Melt Melt Melt L-LD(%) 88.5 98.3 55 99.5 99.85 99.95 meso-LD (%) 5.2 2.3 12.4 0.5 0.1 <0.1Acidity (meq/kg) 290 90 — — — 3 Yd of L-LD (%) 100 — 14 — — 86

It should be noted that the product resulting from the prepurificationin the aqueous phase can be recycled at any point in the sequence forpurification by melt crystallization.

c) Example Demonstrating the Possibility of Obtaining a PolymerizableProduct on Conclusion of the Aqueous Prepurification Stage

A sample of crude lactide product comprising 79.1% of L-LD and 9.2% ofmeso-LD will be subjected to an aqueous prepurification treatment.

25% by weight of cold water are added to 1.520 kg of crude product at80° C. The mixture is rapidly brought to its crystallization temperatureand will remain there for 30 min. In order to promote the growth of thecrystals, seeding is carried out using pure lactide crystals.Subsequently, the temperature is reduced to 25° C.

The mixture is subsequently centrifuged and 915 g of large whitecrystals (of approximately 0.65 m) are collected and dried. The analysisof the dried product appears in Table V.

TABLE V After drying L-LD (%) 95.2 meso-LD (%) 4.5 Acidity (%) 0.2 Water(ppm) 200

A small amount (5 g) of the dried product resulting from this treatmentwas mixed with 5 g of L-LD obtained by melt crystallization (cf. TableIV, step 4: L-LD 99.95%; acidity 3 meq/kg; water 47 ppm). This mixturewas introduced into a test tube while flushing with nitrogen. Afterdissolving the mixture (100° C.), a tin octoate solution was added so asto observe a monomer/catalyst molar ratio of 4500. Once the solution waswell homogenized, it was immersed in an oil bath, the temperature ofwhich was thermostatically controlled at 180° C.

After synthesizing for one hour, the test tube was removed and broken,so as to recover a very rigid and opaque polymer. The polymer obtainedwas analysed by GPC in chloroform at 35° C.: its distribution ofmolecular masses weighted by weight was 68 000 (Mw with PS calibrationcorrected on an absolute basis using universal calibration(KPS=1.67×10⁻⁴, aPS=0.692, KPLA=1.05×10⁻³, aPLA=0.563).

d) Example Demonstrating the Effectiveness of the Process Starting froma Crude Lactide Product Synthesized from a Lactic Acid Ester

An amount of ethyl lactate, sold by Galactic under the “Galaster EL 97”label and characterized by a concentration of ethyl ester of 97%, of 20liters is fed to the plant described in Example b. In order to makepossible the transesterification reactions, para-toluenesulphonic acidis added as catalyst at a concentration of 0.5% by weight. Thetemperature of the heating resistors and in the liquid is regulated soas to avoid any difference of greater than 20° C. and to prevent themaximum temperature from exceeding 175° C. To facilitate the rapidextraction of the volatile compound and to prevent excessively greatentrainment of the ester in the distillates, the procedure as in Exampleb will be followed.

After reacting for 10 h, 7.8 kg of distillates characterized by an ethyllactate content of 3% were collected, along with 12.6 kg of an oligomercharacterized by a molecular mass of 960 and a content of L isomer of97.1%.

The oligomer obtained above is treated as in Example b but 1.5% byweight of tin octoate are added and then the flow rate of the mixture isset at 2 kg/h, while the temperature of the liquid “coolant” ismaintained between 85 and 95° C. The crude lactide product is collectedat the outlet of the condenser at a flow rate of 1.78 kg/h, has anL-lactide content varying between 73 and 78% and has a meso-lactidecontent varying between 2 and 5%.

A sample of the crude lactide product obtained above (750 g), comprising75.3% of L-LD, 2.3% of meso-LD and a residual acidity of 83 meq/kg, istreated by following a procedure identical to that set out in Example b.

The enriching of the intermediate fractions and the overall yield bymass of L-LD of the operation are shown in Table VI.

TABLE VI Step 1 Step 2 Step 3 Feed Melt Drain Melt Melt L-LD (%) 75.398.4 43.7 99.4 99.8 meso-LD (%) 2.3 0.3 5.1 <0.1 <0.1 Acidity (meq/kg)83 27 — — 4 Yd of L-LD (%) 100 — 26.7 — 73.0

The L-LD and meso-LD contents are determined by GC after silylation ofthe carboxyl compounds. The acidities are assayed by potentiometry withtetrabutyl-ammonium hydroxide (TBAH) in an anhydrous solvent. The watercontents were determined by Karl Fisher.

The drains resulting from the first steps were mixed so as to obtain amixture comprising 42.3% of L-LD and 5.2% of meso-LD. It will besubjected to a prepurification, before being mixed with the productresulting from the depolymerization for purification by meltrecrystallization identical to that of Example b.

25% by weight of cold water are added to 1.050 kg of crude product at80° C. and the procedure described in Example b was repeated. Themixture is subsequently centrifuged and 397 g of large white crystals(of approximately 0.85 mm) are collected and dried under vacuum at 45°C. The analysis of this product after drying appears in Table VII.

TABLE VII After drying L-LD (%) 92.8 meso-LD (%) 4.3 Water (ppm) 385

The dried product resulting from this treatment, after being mixed withthe product resulting from the depolymerization, will be subjected toseveral steps of purification by melt recrystallization in accordancewith Example b.

Table VIII shows an increase in the yield and in the effectiveness ofthe melt purification.

TABLE VIII Step 1 Step 2 Step 3 Feed Melt Drain Melt Melt L-LD (%) 79.299.1 42.7 99.8 99.9 meso-LD (%) 2.5 0.3 5.9 <0.1 <0.1 Acidity (meq/kg)87 20 — 9 1 Yd of L-LD (%) 100 — 20.2 — 80

It should be noted that the product resulting from the prepurificationin the aqueous phase can be recycled at any point in the sequence forpurification by melt crystallization.

1. A process for the production and purification of lactide, wherein,staffing from an aqueous solution of lactic acid or lactic acidderivatives, the stages comprise: a) evaporation of free water and aportion of the water of constitution until oligomers having a molecularmass of between 400 and 2000 amu, a total acidity as lactic acidequivalent of between 119 and 124.5% and an optical purity, expressed asL-lactic acid, of between 90 and 100% are obtained; b) feeding a mixturecomprising a depolymerization catalyst and the oligomers obtained instep a) to a depolymerization reactor to produce: b1) a lactide-richvapor phase, and b2) an oligomer-rich liquid residue; c) selectivecondensation of the lactide-rich vapor with recovery, in the liquidform, of a crude lactide product freed from the volatile compounds; d)melt crystallization of the crude lactide product to produce: d1) apurified lactide fraction having a residual acidity of less than 10meq/kg, a water content of less than 200 ppm, a total lactide content of99-99.9% and a meso-lactide content of less than 1%; and d2) at least afirst residual fraction having a lactide content comprise between 35 and80%; and e) separating the purified lactide from the step d1) and the atleast first residual fraction from the step d2); and f) aqueoustreatment of said at least first residual fraction from the step d2) ofmelt crystallization, consisting of: f1) extractive and controlledcrystallization of the residue fractions in an aqueous medium, withcontrol of the geometry of crystals formed and with segregation of alactide suspension towards a solid phase and of impurities towards aliquid phase to carry out aqueous extraction of the impurities; f2)separation of the suspension of crystals formed in step e1) from theliquid phase and then draining to separate a wet cake rich in lactidecrystals from a liquid phase depleted in lactide and laden withimpurities; and f3) drying the wet cake to provide prepurified lactideobtained from said at least first residual fraction of said step d2). 2.A process for the production of polylactide, wherein the stages ofproduction and of purification of lactide, starting from an aqueoussolution of lactic acid or lactic acid derivatives, comprise: a)evaporation of free water and a portion of the water of constitutionuntil oligomers having a molecular mass of between 400 and 2000 amu, atotal acidity as lactic acid equivalent of between 119 and 124.5% and anoptical purity, expressed as L-lactic acid, of between 90 and 100% areobtained; b) feeding a mixture comprising a depolymerization catalystand the oligomers obtained in step a) to a depolymerization reactor toproduce: b1) a lactide-rich vapor phase, and b2) an oligomer-rich liquidresidue; c) selective condensation of the lactide-rich vapor withrecovery, in a liquid form, of a crude lactide product freed fromvolatile compounds; d) melt crystallization of the crude lactide productformed in step c) to produce: d1) a purified lactide fraction having aresidual acidity of less than 10 meq/kg, a water content of less than200 ppm and a meso-lactide content of less than 1%; and d2) at least afirst residual fraction having a lactide content comprise between 35 and80%;and e) separating the purified lactide from the step d1) and the atleast first residual fraction from the step d2); and f) aqueoustreatment of said at least first residual fraction from the step d2) ofmelt crystallization, consisting of: f1) extractive and controlledcrystallization of the residual fractions in an aqueous medium, withcontrol of the geometry of crystals formed and with segregation of alactide suspension towards a solid phase and of impurities towards aliquid phase to carry out aqueous extraction of the impurities; f2)separation of the suspension of crystals formed in step e1) from theliquid phase and then draining to separate a wet cake rich in lactidecrystals from a liquid phase depleted in lactide and laden withimpurities; and f3) drying the wet cake to provide prepurified lactideobtained from said at least one first residual fraction of said stepd2); g) polymerization of the purified lactide fraction obtained in saidstep d1) to polylactide.
 3. The process according to claim 1, whereinthe lactic acid derivatives comprise lactic acid esters.
 4. The processaccording to claim 1, wherein the lactic acid derivatives comprise amixture of lactic acid and one or more lactic acid esters.
 5. Theprocess according to claim 1, wherein the crude lactide product isenriched in prepurified lactide fractions originating from the aqueoustreatment of the residual fractions from the step of meltcrystallization.
 6. The process according to claim 1, wherein theprepurified lactide resulting from the aqueous treatment can be recycledat any point during the production and purification of lactide.
 7. Theprocess according to claim 1, wherein a content of D-lactide during theprocess is controlled by polymerization by ring opening of theprepurified lactide.
 8. The process according to claim 1, wherein theprepurified lactide obtained from the aqueous treatment e) of theresidual fractions d2) exhibits a residual water content of between 50and 1000 ppm, a total lactide content of between 70 and 99%, a contentof lactic acid and lactic acid oligomers of between 0 and 5% and ameso-lactide content of between 0 and 15%.
 9. The process for theproduction of polylactide according to claim 2, wherein thepolymerization of at least one of the purified lactide and theprepurified lactide comprises the steps of: a) addition of a catalyst ora mixture of catalysts to the lactide to form a mixture; b) initiationof the prepolymerization with addition to the mixture formed in step a)of optional comonomers, oligomers, prepolymers, stabilizers, fillers,reinforcing agents or polymerization moderators; and c) polymerizationin an extruder with addition of optional comonomers, oligomers,prepolymers, stabilizers, fillers, reinforcing agents or polymerizationmoderators.
 10. The process for the production of polylactide accordingto claim 2, wherein the polymerization of at least one of the purifiedlactide and the prepurified lactide comprises the steps of: a) additionof a catalyst or a mixture of catalysts to the lactide to form amixture; b) polymerization in an extruder with addition to the mixtureformed in step a) of optional comonomers, oligomers, prepolymers,stabilizers, fillers, reinforcing agents or polymerization moderators.11. The process for the production of polylactide according to claim 2,wherein, during the purification and the production of polylactide, therecycled fractions of lactic acid or the lactic acid derivatives areintroduced in the purification stage of the process for the productionof lactic acid or the lactic acid derivatives.
 12. The process for theproduction of lactide according to claim 1, wherein, during theproduction and the purification of lactide, the recycled fractions oflactic acid or the lactic acid derivatives are introduced in thepurification stage of the process for the production of lactic acid orthe lactic acid derivatives.
 13. The process according to claim 2,wherein the lactic acid derivatives comprise lactic acid esters.
 14. Theprocess according to claim 2, wherein the lactic acid derivativescomprise a mixture of lactic acid and one or more lactic acid esters.15. The process according to claim 2, wherein the crude lactide productis enriched in prepurified lactide fractions originating from theaqueous treatment of the residual fractions from the step of meltcrystallization.
 16. The process according to claim 2, wherein theprepurified lactide resulting from the aqueous treatment can be recycledat any point during the production and purification of lactide.
 17. Theprocess according to claim 2, wherein a content of D-lactide during theprocess is controlled by polymerization by ring opening of theprepurified lactide.
 18. The process according to claim 2, wherein theprepurified lactide obtained from the aqueous treatment f) of theresidual fractions d2) exhibits a residual water content of between 50and 1000 ppm, a total lactide content of between 70 and 99%, a contentof lactic acid and lactic acid oligomers of between 0 and 5% and ameso-lactide content of between 0 and 15%.
 19. The process for theproduction of polylactide according to claim 9, wherein, during thepurification and the production of polylactide, the recycled fractionsof lactic acid or the lactic acid derivatives are introduced in thepurification stage of the process for the production of lactic acid orthe lactic acid derivatives.
 20. The process for the production ofpolylactide according to claim 10, wherein, during the purification andthe production of polylactide, the recycled fractions of lactic acid orthe lactic acid derivatives are introduced in the purification stage ofthe process for the production of lactic acid or the lactic acidderivatives.
 21. The process according to claim 3, wherein the crudelactide product is enriched in prepurified lactide fractions originatingfrom the aqueous treatment of the residual fractions from the step ofmelt crystallization.
 22. The process according to claim 4, wherein thecrude lactide product is enriched in prepurified lactide fractionsoriginating from the aqueous treatment of the residual fractions fromthe step of melt crystallization.
 23. The process according to claim 3,wherein the prepurified lactide resulting from the aqueous treatment canbe recycled at any point during the production and purification oflactide.
 24. The process according to claim 4, wherein the prepurifiedlactide resulting from the aqueous treatment can be recycled at anypoint during the production and purifaction of lactide.
 25. The processaccording to claim 5, wherein the prepurified lactide resulting from theaqueous treatment can be recycled at any point during the production andpurification of lactide.
 26. The process according to claim 3, wherein acontent of D-lactide during the process is controlled by polymerizationby ring opening of the prepurified lactide.
 27. The process according toclaim 4, wherein a content of D-lactide during the process is controlledby polymerization by ring opening of the prepurified lactide.
 28. Theprocess according to claim 5, wherein a content of D-lactide during theprocess is controlled by polymerization by ring opening of theprepurified lactide.
 29. The process according to claim 6, wherein acontent of D-lactide during the process is controlled by polymerizationby ring opening of the prepurified lactide.
 30. The process according toclaim 3, wherein the prepurified lactide obtained from the aqueoustreatment f) of the residual fractions d2) exhibits a residual watercontent of between 50 and 1000 ppm, a total lactide content of between70 and 99%, a content of lactic acid and lactic acid oligomers ofbetween 0 and 5% and a meso-lactide content of between 0 and 15%. 31.The process according to claim 4, wherein the prepurified lactideobtained from the aqueous treatment f) of the residual fractions d2)exhibits a residual water content of between 50 and 1000 ppm, a totallactide content of between 70 and 99%, a content of lactic acid andlactic acid oligomers of between 0 and 5% and a meso-lactide content ofbetween 0 and 15%.
 32. The process according to claim 5, wherein theprepurified lactide obtained from the aqueous treatment f) of theresidual fractions d2) exhibits a residual water content of between 50and 1000 ppm, a total lactide content of between 70 and 99%, a contentof lactic acid and lactic acid oligomers of between 0 and 5% and ameso-lactide content of between 0 and 15%.
 33. The process according toclaim 6, wherein the prepurified lactide obtained from the aqueoustreatment f) of the residual fractions d2) exhibits a residual watercontent of between 50 and 1000 ppm, a total lactide content of between70 and 99%, a content of lactic acid and lactic acid oligomers ofbetween 0 and 5% and a meso-lactide content of between 0 and 15%. 34.The process according to claim 7, wherein the prepurified lactideobtained from the aqueous treatment f) of the residual fractions d2)exhibits a residual water content of between 50 and 1000 ppm, a totallactide content of between 70 and 99%, a content of lactic acid andlactic acid oligomers of between 0 and 5% and a meso-lactide content ofbetween 0 and 15%.